The 26-year-old billionaire, whose brainchild was launched while he was a Harvard undergrad, favors T-shirts, eschews formalities and doesn't even have an office at the company's Palo Alto, Calif., headquarters. But here's why the venerable mag gave him its highest honor:

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“For connecting more than half a billion people and mapping the social relations among them; for creating a new system of exchanging information; and for changing how we all live our lives, Mark Elliot Zuckerberg is TIME’s 2010 Person of the Year," Time editors wrote.

Zuckerberg, the son of a suburban New York dentist, joins a host of commander-in-chiefs who have received the honor, including Presidents Obama and Clinton. Other tech bigwigs to have graced Time Magazine’s cover include Amazon founder Jeff Bezos and Microsoft chief Bill Gates.

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Zuckerberg was a 19-year-old sophomore when he started a Thefacebook.com from his dorm room. It was originally billed as "an online directory that connects people through social networks at colleges." This year, Facebook went over 550 million members, and the site accounts for 1 out of 4 American page views.

Time also points out that if Facebook were a country, it would be the third largest, behind only China and India. Nearly half of all Americans have a Facebook account, yet 70% of Facebook users live outside the U.S.

"We have entered the Facebook age, and Mark Zuckerberg is the man who brought us here," wrote Time.

Zuckerberg, who earlier this year donated $100 million to the Newark, N.J., school system, is worth an estimated $6 billion. Although he initially quibbled with his portrayal in the film "The Social Network," he later led a company outing to see the movie and even joked about it.

His father, a Dobbs Ferry, N.Y., dentist who calls himself "Painless Dr. Z," said young Mark was "strong-willed and relentless."

"For some kids, their questions could be answered with a simple yes or no," Ed Zuckerberg said. "For Mark, if he asked for something, yes by itself would work, but no required much more. If you were going to say no to him, you had better be prepared with a strong argument backed by facts, experiences, logic, reasons."